In magnetic disk drives, data is written and read by magnetic transducers called “heads.” The magnetic disks are rotated at high speeds, producing a thin layer of air called an air bearing surface (ABS). The read and write heads are supported over the rotating disk by the ABS, where they either induce or detect flux on the magnetic disk, thereby either writing or reading data. Layered thin film structures are typically used in the manufacture of read and write heads. In write heads, thin film structures provide high areal density, which is the amount of data stored per unit of disk surface area, and in read heads they provide high resolution.
The present invention is directed generally to devices that can be used, in some implementations, as heads for disk drives, and more particularly the present invention is directed to magnetic tunnel junction (MTJ) devices. An MTJ device has at least two metallic ferromagnetic layers separated by a very thin nonmagnetic insulating tunnel barrier layer, wherein the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. The high magnetoresistance at room temperature and generally low magnetic switching fields of the MTJ renders it effective for use in magnetic sensors, such as a read head in a magnetic recording disk drive, and nonvolatile memory elements or cells for magnetic random access memory (MRAM).
In a MTJ device, one of the ferromagnetic layers has its magnetization fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the field of the other ferromagnetic layer is “free” to rotate in the presence of an applied magnetic field in the range of interest of the read head or memory cell.
To increase both sensitivity and output, the free layer may be established by two sublayers that are separated from each other by an antiparallel coupling. (AP-coupling) layer. Since the magnetic fields of the two free sublayers are antiparallel to each other, the effective magnetic thickness of the overall free layer that is established by the two sublayers is the difference in thickness between the sublayers.
To make an AP-coupled MTJ device, a lower free sublayer is established on the barrier that overlays the above-mentioned pinned stack, then the AP-coupling layer is deposited on the first free sub-layer, then an upper free sublayer is established on the AP-coupling layer. Next, the free layers are formed into a stack by protecting only the area of the free layer sought to be maintained and ion milling the remainder away, down to the barrier covering the pinned stack.
As critically recognized herein, during the above process the barrier might be unintentionally eroded because it is difficult to stop removing material exactly as the last of the free layer intended to be removed is indeed milled away. This results in a deleterious loss of tunnel magnetotresistance between the free and pinned stacks from shunting caused by a breakdown in the barrier and/or by redeposited material.
The present invention makes the additional critical observations. As understood herein, it is necessary for stabilization purposes to provide stabilization structure in MTJ devices, and one way to do this is to surround the free stack with a hard bias material. The present invention recognizes that for optimum stabilization, when so doing the hard bias layer ideally is centered around the free stack.
With these recognitions in mind, the invention herein is provided.